- Pogo oscillation
Pogo oscillation is the term for a potentially dangerous type of
oscillationfound in rocket engines. This oscillation results in variations of thrustfrom the engines, generally caused by variations in fuel flow rate, and placing stress on the frame of the vehicle. Although the term is frequently written POGO, it is not an acronym, but in fact a reference to the bouncing of a pogo stick.
There are several causes of pogo oscillation. The main one is when a surge in engine
pressureincreases back pressureagainst the fuelcoming into the engine, reducing engine pressure, causing more fuel to come in and increasing engine pressure again. If the cycle happens to match a resonance frequency of the rocket then dangerous oscillations can occur through positive feedback, which can in extreme cases tear the vehicle apart.
The most famous pogo oscillation was in the
Saturn VS1-C Stage, caused by the cruciform thrust structure. This structure was an "X" of two I-beams, with an engine on the end of each beam and the center engine at the intersection of the beams. The center of the cruciform was unsupported, so the central F-1 engine caused the structure to bend upwards. The "pogo" oscillation occurred when this structure sprung back, lengthening the center engine's fuel line below (which was mounted down the center of the cruciform), temporarily reducing the fuel flow and thus reducing thrust. At the other end of the oscillation, the fuel line was compressed, increasing fuel flow - causing a sinusoidal thrust oscillation during the 1st stage ascent.
If the oscillation is left unchecked, failures can result. One case occurred in the middle engine of the second stage of the
Apollo 13lunar mission. Fortunately in this case the engine shut down before the oscillations could cause damage to the vehicle. Later events in this mission overshadowed the pogo problem. Pogo was also the cause of some of the problems experienced by Apollo 6. There are other cases during un-manned launches in the 50s and 60s where the pogo caused catastrophic launch failures.
However, modern vibration analysis methods can account for the pogo oscillation to test that it is far away from the vehicle's resonant frequencies. Suppression methods include damping mechanisms or bellows in propellant lines. The
Space Shuttle Main Engines each have a damper in the LOXline, but not in the hydrogenfuel line.
One of the Soviet Union's
N-1 rockettest flights was destroyed due to pogo oscillations on November 22, 1972. Pogo oscillation at stage 1 initial engine cutoff. The launch vehicle was destroyed 107 seconds after liftoff.
* [http://yarchive.net/space/rocket/pogo.html Sci.space.shuttle newsgroup discussions of pogo]
* [http://www.hq.nasa.gov/office/pao/History/SP-350/ch-3-5.html The Perils of Pogo]
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